An 'extraordinarily rare' type of twins created from one egg and two sperm have been identified by medical experts.
The unnamed brother and sister, who are deemed 'semi-identical' by scientists, are now 'beautiful and healthy' four year olds.
Their 28-year-old mother's egg was fertilised naturally by two sperm from the same father – one carrying an X chromosome and one with a Y, researchers revealed.
At first doctors in Australia assumed the pair were identical, but then realised they were different sexes, which is impossible for identical twins.
The phenomenon is extremely rare because these babies often do not survive in the womb – this is believed to be only the second time it has been recorded.
At first, doctors wrongly assumed the foetuses were identical twins because an ultrasound taken early in the pregnancy showed them sharing the same placenta.
But a later ultrasound showed they were different sexes and medics realised something extraordinary had happened.
Experts at the Queensland University of Technology said, after fertilisation, the egg split into three embryos with DNA from two sperm.
One of the embryos did not develop but the remaining two went on to grow as twins, each with different genetic material from the father.
The twin boy and girl, who live in Brisbane, were found to share exact copies of their mother's DNA, but only 78 per cent of their paternal DNA matches.
The girl has already had her ovaries removed because doctors were concerned she might be particularly vulnerable to cancer.
'It turned out that the girl just had some changes in her ovary that people weren't comfortable with, so unfortunately she had to have her ovaries out,' said Dr Michael Gabbett.
'The boy is continuing to have his testes monitored,' he added, saying doctors are using ultrasound scans to track the boy's development.
'Otherwise,' Dr Gabbett said, 'the two twins are beautiful kids, well and healthy.'
The only other reported instance of semi-identical – sesquizygotic – twins was in 2007, and that wasn't identified until after birth.
They were brought to the attention of doctors because one had 'ambiguous' genitalia.
The case was revealed in a paper in the New England Journal of Medicine, which confirmed the phenomenon is rare but could not estimate how often it happens.
Almost all twins are either fraternal – in which two eggs and two sperm create two separate embryos, or identical – in which one embryo is fertilised by one sperm and splits into two embryos.
'This is confirming there is this third type of twinning where it's not fraternal and it's not identical. It's this strange place in between,' Dr Gabbett said.
All sperm carry DNA from the father, but different sperm contain different sections, which is how siblings look different and can be different sexes.
A man's genome – his entire set of DNA – is spread throughout millions of sperm but only a fragment can be passed on to one child.
For example, men have XY chromosomes – some of his sperm contain X chromosomes and some Y, and which sperm fertilises the egg will dictate the sex of the baby (X produces a girl, Y a boy).
To test whether the phenomenon might be more common than doctors believed, Dr Gabbett's team examined an international database of 968 fraternal twins and their parents, but none showed the same pattern.
Source: The Daily Mail